Selector switch having slow-acting relay in magnet circuit



5. RUDEFORTH SELECTOR SWITCH HAVING SLOW-ACTING Nov; 20, 1951 RELAY IN MAGNET CIRCUIT 2 SHEETS-SHEET 1 Filed Sept. l2, 1947 mozEam EImOZ JED 325? n! J lT 1N! 'ENTOR. STANLEY RUDEFORTH mwzotim ozawomzm E ATTORNE Y Nov. 20, 1951 s. RUDEFORTH 2,575,342

- SELECTOR SWITCH HAVING SLOW-ACTING RELAY IN MAGNET CIRCUIT Filed Sept. 12, 1947 2 swims-SHEET 2 VERTICAL AGNET VERTICAL OFF-NORMAL SPRINGS 2 -288 Z m g m L? a a: 2 m 12 '81 L L l l 1 l l l l l l w Q N o m w v N 0 N N N N N INVENTOR.

PERCENT MAKE STANLEY RUDEFORTH ATTORNEY Patented Nov. 20, 1951 SELECTOR SWITCH HAVING SLOW-ACTING RELAY IN MAGNET CIRCUIT Stanley Rudeforth, North Wemblcy, England, as-

signor to Automatic Electric Laboratories Inc., Chicago, 111., a corporation of Delaware Application September 12, 1947, Serial No. 773,673 In Great Britain November 21, 1948 1 Claim. (01. 179-18) This invention relates to improvements in the operating characteristics of electromechanical devices having a slow acting feature and, in par ticular, to those devices which are required to respond and hold during the passage of a train of impulses, as for example, telephone type relays of the kind used in step-by-step automatic telephone systems. In a well-known selector circuit using this type 'of relay the holding function of the relay is secured by short-circuiting a secondary winding at an appropriate time. It will be shown that the maintenance of this short-circuit during ener- 'gizing periods is detrimental to the subsequent holding action of the relay, and it is, therefore, an object of the invention to provide circuit means which dispense with the short-circuit during energizing periods. A further object of the invention is to provide a system whereby the holding relay will fail to hold only on energizing pulses of such short duration that they would fail to actuate associated pulse-operated mechanisms anyway. Furthermore, the normal design of the relay may now be altered to give an even greater gain in performance. It is also an object of the invention to enable a smaller nominal release lag time to be stipulated for the relay so that subsequent switching operations can be made to occur in a shorter time. According to the main feature of the invention, it is arranged that the short circuit applied to the relay winding for the purpose of obtaining a'longer release lag, is interrupted during the period of energization of the relay.

='According to a second feature of the inven-- tion, the interruption of the short circuit is automatically ensured, without affecting its eillcacy during the non-energization periods, by means of a series rectifier.

According to a third feature of the invention, the interruption of the short circuit is automatically ensured, without affecting its eflicacy during-the non-energization periods, bymeans of a-contact, or contacts, suitably related to the main energizing contact.

Aslow release feature on relays may be obtained in one of several ways. The particular method in respect of which the present invention will be described by way of example to enable itto be better understood is that which, for convenience in switching operations, employs two windings on the relay one of which is short cir cuited when it is desired to obtain the slow re lease feature while the other winding is being energized via an impulsing contact. It is a requirement from such relays that they shall initially respond, and subsequently hold, on a train of impulses which may have become distorted in other parts of the system and which may consist of short make pulse periods alternating with relatively long break pulse periods, the make pulse period referring to that period of time the energizing circuit to the relay is closed and the break pulse period referring to that period of time the energizing circuit is opened. At a given impulse frequency, the lower the percentage of make period, basing the percentage on the total of the make time period and break time'period as per cent, on which the relay will hold, the greater is the efficiency of the relay for the purpose.

It is an object of the present invention to show how the efficiency of such relays may be improved substantially. The method by which it is proposed to effect this is best described by reference to a specific practical application of the type of relay under consideration as shown in the accompanying drawings of which Fig. 1 shows only that part of the impulsing element of a type of two-motion selector, such as is at present used in step-by-step automatic telephone exchange systems, which is necessary for an explanation of the invention.

Fig. 2 shows the circuit of Fig. l embodying one means of putting the invention into effect.

Fig. 3 shows tracings of oscillograms relating to the operation of the circuit of Fig. 1.

Fig. 4 shows similar tracings for the circuit Fig. 2.

Fig. 5 shows curves illustrating the improved performance of the circuit according to the in vention.

Fig. 6 shows another means of putting the invention into effect.

Referring now to Fig. 1, when the selector circuit is first seized by preceding switches in the switch train, the circuit is closed to relay A over leads I 00 and till in a manner well-known to those'versed in the art. Relay A operates and closes the make-before-break springs Al A circuit is thus closed from ground at contacts C2 of relay C, through contacts I42 and H3 of the vertical off normal springs I40, through contacts 2 and 3 of spring set Al, through the windingof relay B, resistance YA, and the vertical magnet V to battery. Relay B operates. The verticalmagnet V is pro-energized by this circuit but will not operate in series with resistance YA and relay B. Operation of relay B closes a multiple holding ground to relay B from contacts BI and also closes the circuit to winding ill of relay C through contacts I and 3 of spring set All andthrough the winding I!!! of relay C. The circuit is now in readiness for the receipt oiiatrainof' impulses produced by the alternate closing and opening of contacts I and 3 of spring set AI to impulses received by relay A from the precedin switch train.

Relay A, following the impulses received over line I li3Illl in a well-known manner, causes the contact set Al to successively release and reoperate, closing and openingcontacts I and 3 the first release of relay A, the'con-tacts I and 3 -are closed, shorting the Winding of relay B. The shorting of relay 3 decreases the resistance in the vertical magnet circuit and the pre-energized vertical magnet V operates from ground at contacts BI, through contacts 9 and 3 of relay A and through winding I id of relay C and resistance YA in parallel. The vertical magnet V operates to raise the shaft of the switch mechanism one step and the shaft in taking the first step causes the operation of vertical 01f normal springs I40 in a well-known manner. When relay A restored, it shortedre'lay B as previously states thus rendering relay B slow-to-release. Relay B will not release during the period that relay A has restored and will again be fully energized when relay A reoperates on the next pulse and removes the short from relay B. Operation of the vertical oif normal springs MG closes the ground from contacts BI of relay B through contacts I44 and H to a point between winding III of relay C and resistance YB and battery, thus shorting winding l I I of relay C. Resistance-YB prevents the ground at contacts Bl from causing a direct short of the battery; Short-- ing winding I Ii of relay C renders relay C slow to-release.

Relay A, on operating, removes the short from relay B by opening contacts I and 3 thus closing relay B in the series circuit from ground at contacts BI to the vertical magnet V through resistance YA and, winding Ill! of relay C in parallel; Theraddition of the resistance of the winding of relay 'B' in this circuit causes the vertical magnet V to restore. Relay C being sl0W-to.-release because of the shorted winding. I II does. not immediately release. On the second impulse relay A restores closing contacts I and 3 again shorting relay B and closing the ground at contacts BI through contacts I and 3 through resistance and winding I ill of relay C in parallel to the vertical magnet V. The vertical magnet V again operates to step the switch shaft. shorting relay l3 causes an increase'of current to flow in winding m3 of relay C to thereby hold relay C operated, this increase of current occurring before the slow release relay C has had time to release. The above cycle of operations continues, relay A following the impulses received from the preceding. switch train and in turn pulsing the vertical magnet V to cause the switch mechanism to step. After the last impulse has been received, relay A reoperates, removing the short from. relay B and placing relay B in the series circuit to the vertical magnet through resistance YA and winding II!) of relay C in parallel. The vertical magnet V restores. Slow release relay 0 restores after a short period of time. Relay B is held operated from the ground at contacts BI through relay B and through resistance- YA and vertical magnetV to battery. The release of relay C opens winding II ii and also closes ground at contacts C2 through contacts MI and I42 of verticalofr" normal springs I40 to associate circuit members for subsequent operation not material to the invention contained herein.

; .It will be noticed from the above detailed circuit explanation that the release of relay C during the period when relay A was operated between impulses was sufiiciently delayed by the short circuiting of its winding I I I so that relay C could be reenergized through its winding I I0 when relay A- released on the following impulse. Thus relay C will be held operated during the period of impulsing provided that the per cent make sufiiciently high, the make period referring to that period of time relay A is released and relay B thereby shorted, the break period, being that period of time relay A is operated and relay B thereby connected in series with. winding III! of relay C.

Without the invention about to be described. it sometimes happens thatdistorted impulses received by relay A are such as to cause: failure of relay 0 to hold during i-mpulsing although the same make pulses received by the vertical magnet V are sufficiently long to permit satisfactory operation of the selector mechanism. If, at a given impulse'frequency, the C relay can be made to hold on make pulses shorter than are required for correct operation of the vertical magnet V and mechanism, then the selector will be capable of response to more widely distorted impulses than at present. 7

In order to show how the impulse holding efliciency of a relay, such-as the C relay in Fig. 1, can be improved, consider Fig. 3 comprising tracings from oscillogramsof the currents 21 and i2 inthe C relay windings I I I and Ill], respectively and of-the flux V,'measuredat the pole-face of the: relay. Assume that winding III is short circuited and that the AI break contacts I and 3 are breaking and making due to the receipt. of a train of impulses by relay A. When AI: contacts I and 3 break, the current through winding lIdof relay C falls (point rt in Fig. 3) and a current is" induced in winding I ll in a direction tending to maintain the relay flux. This current, iI, decays exponentially and there is acorresponding decay of flux, but it is still of finite value when the AI contacts I and 3. close and current i2 increases in winding I ID. The flux created by the current induced in winding I II is now ina direction opposing that due to 2'2 in winding III] (point ii in Fig. 3); Thus, the resultant flux rises slowly. It will be appreciated that if the make pulse period is short, and the succeeding break pulse period long, then the flux may not attain its maximum value during the make period and hence will reach the release value'of flux during the succeeding break period earlier than would have been the case had the flux risen rapidly dur-- ing the make pulse period.

It is an object of the invention to eliminate the current induced in winding I Iv I when current is increased in winding i It] so that the flux will more quickly attain its maximum value during the make pulse periods. and will, therefore, take longer to reach the release flux value during. the

succeeding break pulse periods. Figs. 2 and 4 show one way in which this is accomplished by the invention by the insertion of a rectifying element, MR, in that winding, (1 II in the specific example), normally short circuited to secure the slow release feature, in such a direction as to permit energisation of the relay and, at the same time, to eliminate the induced currents in that winding which would otherwise retard the flux rise. The tracings of oscillograms in Fig. 4, strictly comparable with those of Fig. 3, clearly depict the substantial increase in the rate of rise of flux and the higher average value of flux obtained during impulsing caused by the application of the invention.

Another way in which this improvement may be accomplished is shown in Fig. 6 from which it will be seen that a make contact A2 of relay A is inserted in series with the winding II! of relay C. Contact A2 closes to permit the energisation of winding II! when relay A operates, and opens to interrupt the short circuit each time relay A releases, that is to say during each period of increasing current in the H winding of relay C. The short circuit therefore is not available for the circulation of currents which would otherwise be induced in the H l winding as a result of the increasing current in the l 10 winding of relay C.

Fig. gives a further illustration of the effectiveness of the invention in improving the impulse holding emciency of the type of relay described in the specific example. In this figure which depicts curves resulting from plotting impulse frequency in impulses per second against per cent make for satisfactory holding during impulsing, curve I is for the arrangement shown in Fig. 1 with a predetermined static release lag of 100 milliseconds, curve II is for an arrangement according to Fig. 2 with a static release lag of 90 milliseconds and curve III is for the same arrangement with the static release lag increased to 100 milliseconds, the static release lag referring to that period of time that it takes the relay C to release after the current through winding I ID has been reduced by closing relay B in series with the winding H0, assuming that the current is not again increased through winding I I0. It can easily be seen from curves I and II that the application of the invention reduces of the relay incorporating the invention be adjusted to have the same static release lag as obtained without the invention, (curve III), a further increased holding efficiency occurs, notably at lower impulse frequencies.

In the specific example which has been described, application of the invention would enable increased dialling ranges between telephone exchanges to be effected.

While the invention, and one method of carrying it out, has been described with particular reference to telephone systems and circuits, it will be appreciated that it can be applied to any mechanism requiring a slow decay of flux during periods of nonenergisation, and with substantial improvement in the time taken to attain a given flux.

I claim:

In a telephone system, a driving magnet for actuating the mechanism of an automatic telephone switch, a relay having a, first winding and a second winding in inductive relation to said first winding, a source of direct current, a first circuit including said driving magnet, said first winding and said source, a rectifier, a resistance, a second circuit including said second winding, said rectifier, said resistance and said source, a third circuit for short circuiting said second winding including said rectifier and excluding said resistance and said source, means for completing said second circuit to operate said relay, means for intermittently increasing the current flow in said first circuit to maintain said relay operated and to intermittently operate said driving magnet, means operated by said driving magnet on initial operation thereof to complete said third circuit, and thereby short circuit said second winding and disable said second circuit, said rectifier connected in said second and third circuits so as to prevent the induction of current in the second winding due to the increase of current in said first winding.

STANLEY RUDEFORTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,742,367 Nettleson Jan. 7, 1930 2,137,423 Taylor et al. Nov. 22, 1938 2,390,666 Rees Dec. 11, 1945 2,421,148 Hadfield May 27, 194'! 

